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uRADMonitor â€“ Online remote radiation monitoring station

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Proud Mary

Wed Jan 11 2012, 01:27PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

I'm sorry to hear about your G21H tube. I thought it looked a very nice one.

Now, as to the service life of GM tubes.

1) Life expectancy expressed as total counts:

(a) the obsolete (1940s) argon-ethanol GM tubes had a short service life, because some of the ethanol quench gas was used up with every avalanche, and was not replaced. So we will assume our talk is about halogen tubes only.

(b) in amateur use, where our strongest sources are likely to be natural uranium radiominerals such as pitchblende (< 100Î¼Gy/hr at 30mm distance) the manufacturer's total counts lifetime will never be reached.

2) Life expectancy in the Time domain.

(A) structural failure including:

(i) seal failure in mica window tubes - something quite common in older devices. This may not be visible to the naked eye.
(ii) porosity in glass metal seals.
(iii) slow chemical attack on metal elements by halogen gases due to poor design choice of metals.

(B) Growing quietly old, carefully stored in the dark e.g. in an unopened box. In my experience, there is a very good chance that a tube that has never been used and is still in an unopened box will still perform in accordance with its data sheet written 50 years earlier. I have tested many GM tubes from the 1950s and 1960s, and not yet found a bad one among tubes still in their original boxes with their original paperwork. There will be some bad ones, no doubt, but not very many*.

Avoid buying tubes from experimenters who may have cruelly abused them. For example, the properties of a tube run for any length of time at excess voltage (i.e. above the Geigerâ€“MÃ¼ller region of the slope) may have been permanently changed.

Lastly, if you remove the opaque coating from a glass GM tube to see what is inside, be sure to repaint it before using the tube anywhere where light can get at it. The opaque coating is there to block UV photons, which can and will cause ionisation events in the gas - usually seen as spotty, erratic counting.

* "The shelf life of Geigerâ€“MÃ¼ller tubes, when out of commission or when installed in equipment that is itself out of commission, is extremely long." - Centronic's Geigerâ€“MÃ¼ller Tube Theory see:

radhoo

Wed Jan 11 2012, 05:51PM

Registered Member #1938 Joined: Sun Jan 25 2009, 12:44PM
Location: Romania
Posts: 699

Thank you. Indeed the Centronic paper on GM tubes was a very good read, I had to print it so I could underline important paragraphs with the pencil.

I do have a G21H as well and it is in perfect shape (NOS) , the tubes that is damaged and I mentioned before is G15H . I got it from the atomic institute directly, so it was only used in adequate equipment. It's the small sized type not more than 10cm in length, so was probably used with stronger sources.

It doesn't show in the video above.

Proud Mary

Wed Jan 11 2012, 07:28PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

radhoo wrote ...

...the tubes that is damaged and I mentioned before is G15H . I got it from the atomic institute directly, so it was only used in adequate equipment. It's the small sized type not more than 10cm in length, so was probably used with stronger sources.

An atomic institute could probably reach the maximum count life of G15H very quickly indeed!

Don't let the grid of G15H float, or it will collect an unwanted charge. As a quick experiment, you could connect the grid directly to the cathode and see what happens.

If you are sure you have connected it up correctly, then I would spend no more time and money on it. Save the mica for something else. (I have used the mica sheets which you can buy new on ebay as windows for wood-burning stoves. They are quite thick, so you can split them down if you are careful.)

radhoo

Sun Sept 02 2012, 10:38AM

Registered Member #1938 Joined: Sun Jan 25 2009, 12:44PM
Location: Romania
Posts: 699

[moved]

radhoo

Sun Sept 02 2012, 09:51PM

Registered Member #1938 Joined: Sun Jan 25 2009, 12:44PM
Location: Romania
Posts: 699

I hooked up the circuit to an atmega8 with a Ethernet interface based on the enc28j60 chip, presented here:

I added a few lines to the uC software, to have a T0 counter and a T1 timer (on the Atmega8). This way I was able to measure the pulses , the passing seconds , and have a quick way to compute the CPM.

By using the tube's datasheet information, it would be also possible to extrapolate a function for computing the dose in uSv/h.

So first tests are a success, will report back later. Experimental setup looks like this:

Proud Mary

Sun Sept 09 2012, 12:13PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

radhoo wrote ...

By using the tube's datasheet information, it would be also possible to extrapolate a function for computing the dose in uSv/h.

Hi Radhu old friend!

The summer is over at last, and now I am re-decorating Haus Stella before I can return to my winter experiments.

Here are a few useful features you might want to add to your programme menu, if you haven't already thought of them all:

Dead time compensation. Here you take the dead time from the datasheet and use it to calculate the number of counts that statistics predict would have occurred during the dead time intervals, and then add them automatically to the total counts. This becomes increasingly important at counting speeds over about 100 c.p.s.

Pulse pile-up compensation. Another essential for high counting speeds. Pile-up happens when pulses occur so close together that one pulse has not fully collapsed before the next one begins. A discriminator circuit is usually used here. The pile-up problem can be reduced by keeping anode capacitance as low as possible and then by electronic quenching

Total counts per hour, up to - for example - ten days. This is how you can detect small changes in the radioactive background, and do experiments which measure small effects, like determining the activity of potassium salts and other weak emitters.

Integration - average c.p.m. etc

A buffered data output to a PC

Saturation alarm: if the tube becomes paralyzed by high dose rate as in an X-ray beam.

Auto-Deduct tube's own background. This is the figure often specified in Russian datasheets, and is the counting rate when the tube is placed inside a very heavy lead chamber lined with aluminium. It is a very important figure at low count rates, as it can often exceed the true background. There is a very good but old paper on this subject here:

I look forward to seeing your next developments with this great project!

radhoo

Sun Sept 09 2012, 02:43PM

Registered Member #1938 Joined: Sun Jan 25 2009, 12:44PM
Location: Romania
Posts: 699

Hi Proud Mary, good to see you back on 4HV, guess you've had a nice summer holiday.

Thanks for your excellent suggestions, I was familiar with some of the ideas (that centronics material really helped me, wish it was more than a single chapter) as for the rest, it's great to see how much can be improved with the correct approach.

I'll see how to integrate as many as possible of these good ideas.

Currently I'm changing the HV supply from my feedback controlled transistor version, to a PWM driven inverter, so the microcontroller would do everything on its own with little additional components. Thanks to an oscilloscope that I've finally acquired, I'll be able to keep ripple to a minimum, to be sure the tube's got everything it needs.

Proud Mary

Sun Sept 09 2012, 05:56PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

A variable HV supply - 300V to 900V - would be an excellent feature, as this would enable you to change tubes, and set them correctly at the centre of the plateau voltage.

There are some older tubes that need over 900V - including the Blue Romanian Horror and the American aluminium toothpaste tube - but you can ignore these if it would be easier to keep the variable voltage range from 300 - 600V.

No great stability or supply filtering is necessary as the Geiger plateau is very broad, and the tube pulses across the measuring resistor are of the order of 1 - 3V, so they are easy to separate from noise.

The Ð¡Ð˜-22Ð“/SI22G you have there is an excellent gamma tube!

I have a lot to do in the next two weeks, but will have the time to carry on with my low energy experiments then.

radhoo

Tue Sept 11 2012, 10:19AM

Registered Member #1938 Joined: Sun Jan 25 2009, 12:44PM
Location: Romania
Posts: 699

I totally agree, a variable voltage would be useful for using various tubes.
However for this project, I will have to stick to 400V . The project will be boxed and placed on a remote location outside. From there it will collect data and build various graphs. I have the geiger couter , a temperature sensor, a humidity sensor and a light intensity one. If anyone has other ideas on additional hardware that I should pack in, now it's the right time to say it.

A future project might be having a geiger counter dosimeter with multiple tubes, etc, and a nice LCD display to output the data. Or even some kind of "lab-tool" for testing the tubes.

Regarding discrimination, yes, multiple pulses happen, and would be good to measure them correctly. The following pics show a few such pulses, as recorded with the oscilloscope:

A little update on what I did. Yesterday I had to put the microcontroller software together. As I am using an ATMega8, I opted for the following:
- INT0 interrupt to count all incoming pulses on raising edge. I do not have any discriminator yet.
- T0 timer to count passing time. I configured it to generate 1.024ms events, so 976 such events will count for a second.
- T1 timer to generate PWM signal. I got it to accept variable frequency and duty factor. I will probably go for 40KHz to drive a little 2n2222 transistor that pumps the primary of a small ferrite core transformer:

I noticed that without any protection, I will get voltage spikes in the microcontroller circuit. So a fast diode, used as snubber was needed across the primary of the small ferrite transformer . See the comparisor without and with protection:

I still have some issues:
1. I almost used all the program memory available for the atmega8 and still need to code some additional routines. I might be forced to move to a more capable uC. I'll try to optimize and reduce code before I have to do anything like that.
2. There's some interference, between the timer1 PWM, with the ISP interface that handles the Ethernet module:

Because of that, the microcontroller hangs at some times. This is extremely annoying considering I need to have this project as a remote box running for large periods of time.
Some of the factors that are causing this:
- bigger duty cycle (timer1's PWM)
- some delays in the software's main loop
- reading ADC ports : I need to read them for sensors, and for checking the inverter voltage. The inverter produces roughly 400V when powered on. The voltage is read using one ADC port. If the voltage is less then required, the duty cycle is increased. If the voltage is bigger then the target set, the duty cycle is decreased.

I will find the cause of this eventually. As it is now, the software opens a TCP/IP socket on port 80 where it exposes some HTML content. So it is a small WEB Server, where you can see some details such as number of times it was accessed, pings received, time since startup, number of counts, CPM, and the dose in uSv/h , estimated for the SBM-19 tube.
Instead of this functionality, which I needed only for testing, the final software needs to work as a client, not a server. After computing all the sensor data locally (including the radiation dose), it will send it online, to a script running on my webpage. So instead of opening a port and waiting for incoming connections, the idea is to have it open a connecting to my server and export the data there. Easy. I believe the timing issues that currently lock my uC from time to time, will be resolved, as having an ongoing connection instead of keeping a port open is better.

Here's a photo showing the software run on a test board, with a LCD hooked up for showing the dose in uSv/h :)

The final board will not have a display, so this is just for testing.
The software uses a 5 seconds integration to compute the CPM. Each new CPM value is averaged to the last one. For the final version I will probably use a 10second integration, but also a longer time period as a separate result.

Here it shows 444cpm, which for the SBM-19 should be 0.89uSv/h . Can anyone else double check this , please?

The radiation source was a Am241 pellet. The RD1706 shows a similar dose 0.99uSv/h at a similar distance from the source.

radhoo

Fri Sept 14 2012, 09:23PM

Registered Member #1938 Joined: Sun Jan 25 2009, 12:44PM
Location: Romania
Posts: 699

I've been working on the inverter. It seems I'll be able to have it run in a very wide range of values. This is thanks to a new ferrite core transformer that I made: A22 ferite core (it is really small, 1cm diameter), 16 turns primary with 0.2mm wire, and 600 turns secondary with 0.1mm wire. It is driven by a 2n2222 generic transistor, using the 5V rail.
The output is fully rectified using 4 fast diodes set in a bridge. No multiplier required! :)

Yellow/CH1: the PWM signal driving the inverter;s transistor, at PB1 on the Atmega8
Cyan/CH2: the high voltage output after the rectifier bridge

I made some tests with various frequencies. Here are some results:
40KHZ:

duty:12% 332V
duty:15% 344V
duty:16% 396V
duty:17% 404V
duty:25% 672V
duty:35% 792V

20KHz:

duty:10% 392V
duty:15% 616V

15KHz:
duty:6% 352V
duty:8% 440V
duty:10% 488V

10KHz:

duty:1% 200V
duty:6% 380V
duty:16% 768V

I will probably choose a 20KHz frequency to set it out of the audible spectrum.

I'm still having issues when using the enc28J60 code together with the Timer1 PWM code. So either I have the Ethernet interface, either I have the PWM signal for the HV inverter. Sometimes it works, but not very stable. Any ideas?

EDIT: some interesting results, and good news on the same time. Using a 8x prescalling when setting the Timer1 PWM, seems to allow the code to work together with the enc28j60, as expected. So this seems a good starting point, too bad I don't know the exact cause of these issues. Code for enc28j60 is rather complex , already.

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